Methane has been proposed as a fuel for Otto-cycle engines or motors because, like hydrogen, it has high anti-knock qualities and produces an exhaust gas which is particularly free from toxic, noxious or otherwise detrimental components. In fact, like hydrogen, methane has been considered a particularly effective fuel for such internal-combustion engines by contrast with higher hydrocarbons (see for example the German textbook HUTTE, Vol. I, Verlag Wilhelm Ernst, Berlin 1941).
A particular difficulty is encountered, however, in selecting fuels for powering internal-combustion engines for automotive vehicles, namely, the storage of the fuel in sufficient volume and safety to permit wide spread use. For the most part, because of that storage problem, there has been no major shift away from higher hydrocarbons to other fuels potentially constituting more effective and/or environmentally favorable energy sources.
Most potential fuels which are gaseous at room temperature and pressure cannot be stored in sufficient volume and in a limited space to power a vehicle so that it will have a satisfactory range, except under extremely high pressures and at very low temperatures. Such low temperatures are difficult to generate on vehicles and high pressures are dangerous.
Hydrogen, however, has been the subject of considerable research with these problems in mind and the technology for storing hydrogen at relatively low pressures and at noncryogenic temperatures has advanced to a significant degree. For example, hydrogen can be stored in a tank or other vessel by packing the vessel with hydride-forming substances which effectively chemically bond the hydrogen or store the hydrogen as interstitial hydrides within a crystal or grain structure.
The hydrogen is stored in this mass by feeding it under pressure to the tank and is liberated from the mass by reducing the pressure and/or heating the mass. Particularly effective hydride formers are Fe-Ti alloys.
Analogous masses for the storage of methane have not been proposed nor am I aware of any detailed study heretofore of the problem as applied to methane because, apparently, of a prejudice in the art that such approaches would not be successful. Indeed, there has not been any major progress or even detailed study in the use of methane as a fuel for powering the internal-combustion engines of automotive vehicles.
In German patent document (published application-Offenlegungsschrift) DE-OS No. 23 02 403 there are described efforts to permit the storage of gases, among which methane is mentioned, for use in powering automotive vehicles by providing a tank or vessel with an adsorbent for the gas. The adsorbent is capable of picking up methane at low pressures; the stored gas volume can be increased, according to the publication, by 78% with a pressure of 70 kg/cm.sup.2 which could be raised still higher for a further augmentation of storage capacity. These values are so unattractive that the reason for the lack of any significant development in the use of methane as a motor-vehicle fuel is readily apparent.